This invention relates generally to radio frequency circuits and more particularly to frequency conversion circuits such as mixers and multipliers.
As is known in the art, frequency conversion circuits provide an output signal in response to an input signal having a frequency which is higher than or lower than the frequency of the input signal.
Radio frequency mixer circuits are widely employed in so-called superhetrodyne receivers which include in addition to the mixer an intermediate frequency (IF) amplifier tuned to a predetermined frequency provided from the mixer and a frequency detector which is fed the amplified signal from the IF amplifier. Generally, superhetrodyne receivers also include an input broadband amplifier which amplifies an input radio frequency signal typically received from an antenna, for example, and a local oscillator signal source. The received, amplified input signal and local oscillator signal are each fed to the mixer circuit to provide in response thereto an output signal including a pair of frequency components equal to the sum of and difference between the frequencies of the input signal and the local oscillator signal. Typically, the sum frequency component signal is filtered from the output signal and the difference frequency component signal is fed to the IF amplifier.
There are several types of mixers known in the art. One type of mixer is the so-called single-ended mixer which includes a non-linear device such as a transistor or a diode which is fed by the input signal and the local oscillator signal. In response, an output signal is provided having the sum and difference pair of frequencies .vertline..omega..sub.RF .+-..omega..sub.LO .vertline.. One problem with this type of mixer is that the single-ended mixer provides undesired output frequency components such as the original input signal frequency .omega..sub.RF, local oscillator signal frequency .omega..sub.LO, and harmonics of the original input signals, as well as, products of the harmonics of the original input signals (M.omega..sub.LO .+-.N.omega..sub.RF). Filtering or suppression of these signals is generally required. Moreover, with such a mixer, there is no isolation between the LO and RF signals which in certain applications is a problem.
A second type of mixers is a so-called singly balanced mixer which generally includes a pair of single-ended mixers and a 3 dB hybrid coupler. The inputs of the coupler are fed by the input signal and local oscillator signal and outputs of the hybrid are coupled to the input of each single-ended mixer. The output of the mixer elements are combined in a common junction to provide the output IF signal. One of the problems with singly-balanced mixers is the relative difficulty in fabricating the 3 dB hybrid coupler, and in particular, fabricating a 3 dB hybrid coupler which is broadband. Furthermore, it is difficult to fabricate a hybrid coupler as an integrated circuit.
A third type of mixer is the so-called doubly balanced mixer. The doubly-balanced mixer generally includes two singly balanced mixers each fed by the local oscillator signal having a 180.degree. differential phase shift provided by a hybrid coupler or balun and one of a pair of input signals. The outputs of each balanced mixer are combined together by an IF hybrid coupler. One problem with doubly-balanced mixers is that the couplers or baluns are generally difficult to fabricate as monolithic integrated circuits. Accordingly, the double balanced mixer is not easily fabricated as a monolithic integrated circuit. A second problem with a double balanced mixer is the relatively narrow frequency bandwidth of operation due to the presence of couplers and the baluns.
One type of mixer which overcomes some of these problems as well as others is described in U.S. Pat. No. 4,662,000, by Tajima et al., and assigned to the assignee of the present invention. In the above-mentioned patent, a distributed mixer including a plurality of successively coupled dualgate field effect transistors is described.
The above-mentioned patent provides a circuit which overcomes many of the problems mentioned above concerning single-ended, balanced, and double balanced mixers and provides a technique in which relatively large bandwidth is achieved. However, the use of dual-gate field effect transistors typically increases the noise figure of a mixer, since dual-gate transistors tend to having higher noise figures than single gate transistors. In many applications, the higher noise figure of the mixer would not pose a serious problem. Moreover, in such applications, RF signals may be fed by low noise amplifiers to reduce the effect of the mixer noise figure on a final output, IF signal.
Nevertheless, it would be desirable to reduce the number of circuits employed in fabricating mixers and receivers. In particular, it would desirable to eliminate the LO amplifier, input RF amplifier, as well as, output IF amplifier or at least significantly reduced their requirements. Moreover, in certain applications, such as electronic warfare surveillance applications, it is necessary to prevent radiation of the LO signal through the antenna. This requirement necessitates high isolation between LO and RF terminals in particular in the direction LO to RF. In certain instances, isolation requirements beyond -90 dB may be required. Such an isolation specification is not easily achieved with known techniques.
A further problem with many known mixers is that such mixers tend to have low conversion efficiency. This characteristic increases the power requirement for the LO signal, source, and IF amplifiers. Again, reduction of such requirements would be desirable.